High response, multistage servovalves have been used successfully for over four decades. Almost invariably, the first stage of the servovalve is a double jet flapper valve with a torque-motor actuated flapper and requires a high and constant supply pressure, in the 3000 psi range. This pressure range and the requirement for constant pressure are not suitable to many industrial applications. For this reason, the investigation upon which this Application is based had as its initial goal, the use of low pressure, multistage servovalves in systems where the supply pressure could vary between 500 and 3000 psi. This goal has been met through this invention, but it has also been found that the application of the invention to the high pressure, multistage servovalve significantly extends its frequency response and extends the supply pressure range to below 50 percent of design pressure.
Double jet flapper valves which are in wide use employ a torque-motor driven flapper that is placed between two jet nozzles. Each jet nozzle is fed from a pressure source through an orifice. The torque-motor is spring centered to null position. At null position, the flapper is centered between the two nozzles, the nozzle pressure forces are balanced and the spring force and the torque-motor current are essentially zero. When the current through the torque-motor coil and consequently the electro-magnetic force are increased from zero, in either direction, the spring force acts in opposition to the flapper deflection and the deflection produces a nozzle pressure difference that also oppose the deflection. An additional opposing force that may act on the flapper is the second stage feedback force, where used. At a given flapper deflection from the null position, there is a force balance between the forces that act on the flapper: the spring force, the pressure force, the second stage feedback force and the electro-magnetic force.
The pressure difference between the two nozzle that is provided by the orifice fed flapper valve is proportional to the flapper deflection from null and to the supply pressure. A further effect of supply pressure is that the gain of pressure difference to torque-motor current also increases with supply pressure. In pressure control servo systems, the second effect can lead to system instability if the supply pressure is raised. In addition, in flapper valves designed for low supply pressure, as is functional in pressure control systems, the high null flow accompanying a substantial supply pressure increase may cause disruptive pressure drops in flow passages. Flow control servovalves which have been designed for high response operate at relatively high supply pressure, usually 3000 psi. The inlet orifice diameter, jet nozzle diameter and nozzle gap are matched, in design for a given null flow rate and pressure differential range, only at 3000 psi supply pressure. Because of the nonlinearity of the flow-pressure relationship, there can be serious loss in pressure differential if the supply pressure is reduced only 25 percent and an almost complete loss of function at 50 percent. This characteristic sensitivity of both types of servovalves to supply pressure makes them difficult to apply to the recently developed load responsive systems where the supply pressure is varied for efficiency.